In recent years, 3C electronic products, such as notebooks, foldable mobile phones, digital cameras, video cameras etc., have a tendency to become smaller, thinner and lighter. This leads to an increased demand for portable energy source, i.e. “secondary battery”, especially those with miniaturized sizes and lightweight. Furthermore, with the increased demand for electronic products having multi-functions, high speed, high performance and high power, a secondary battery having high electric capacity is also desired.
In general, the energy density of a lithium ion battery is about 260 to 270 kWh/m3, which is about as twice or more as that of a nickel-cadmium alkaline secondary battery. Lithium ion/lithium polymer secondary batteries have the advantages of rapid charge, high-power discharge, high energy density, long cycling life etc. Accordingly, the lithium ion secondary battery and the lithium polymer secondary battery play important roles in the field of the secondary batteries for small electronic products.
The lithium ion secondary batteries and the lithium polymer secondary batteries are based on the same electrochemical principles as the common batteries. They each comprises a positive electrode, a negative electrode, a separating film and an electrolyte solution as main components. Lithium ions move from the positive electrode to the negative electrode during charge process, while moving from the negative electrode to the positive electrode during discharge process. The positive electrode and the negative electrode each comprises a current-collecting metal substrate, and a surface coating layer (also referred to as “electrode film” hereinafter) comprising an electrode active material, a conductive additive and a binder.
In case that the electrode paste composition is used in preparation of the surface coating layer of a positive electrode, the composition usually contains a metal oxide with high density (such as LiCoO2) as the positive electrode active material and carbon or graphite powder with low density as the conductive additive. When these components with different densities are mixed with the binder such as polyvinyl difluoride (PVDF) and dispersed in the solvent such as N-methylpyrrolidone (NMP), sedimentation may occur, which will result in uneven coating. Thus, the dispersability of the electrode paste composition is considered as one of the main factors affecting the stability of the quality of lithium ion batteries and lithium polymer batteries.
On the other hand, the dispersion uniformity of the electrode active material in the surface of the electrode film, the adhesion of the electrode active material to the electrode film, and the adhesion of the electrode film to the current-collecting metal substrate are all important factors affecting the electrical performance of lithium ion batteries and lithium polymer batteries.
Therefore, it is desired to have an electrode paste composition having high dispersability, low viscosity and high storage stability.